Treatment of waste

ABSTRACT

A method of treating inorganic solid waste in a bath of molten metal contained in a vessel (3) which has a space above the bath and a waste gas outlet (11) is disclosed. The method comprises injecting waste into the bath to form a primary reaction zone (13) in which there are reactions between the waste and the bath or in which the waste undergoes a change of phase to convert the waste into more readily recoverable or disposable products. The method further comprises injecting oxygen-containing gas towards the surface of the bath to form a secondary reaction zone (17) in a section above the bath through which oxidisable products released from the primary reaction zone (13) flow to reach the waste gas outlet (11) in the vessel (3) and in which the oxidisable products are oxidised and the heat released by such oxidation is transferred into the bath.

The present invention relates to the treatment of inorganic solid waste.

In particular, the present invention relates to the treatment ofinorganic solid waste which is commonly referred to as "dusts".

The term "dusts" as used herein is understood to mean any relativelyfinely divided particulate material and includes but is not limited to:

(a) metallic or metallic oxide containing material substantiallycollected by gas cleaning systems operated, for example, as a componentof pyro-metallurgical production and processing; and/or

(b) inorganic residues arising from the combination or incineration oftoxic, hazardous and non-hazardous wastes, including fly ash, bottom ashand particulate material collected by gas cleaning systems.

In many instances, for a range of environmental and materials handlingreasons it is difficult and expensive to dispose of dusts. For example,dusts often contain hazardous compounds and require particularprocessing before disposal. In addition, dusts often contain componentswhich, whilst valuable, cannot be recovered economically.

It is an object of the present invention to provide a method of treatingdusts which alleviates the disadvantages described in the precedingparagraphs.

According to the present invention there is provided a method oftreating inorganic solid waste in a bath of molten metal contained in avessel which has a space above the bath and a waste gas outlet, themethod comprising:

(a) injecting waste into the bath to form a primary reaction zone inwhich there are reactions between the waste and the bath or in which thewaste undergoes a change of phase to convert the waste into more readilyrecoverable or disposable products; and

(b) injecting oxygen-containing gas towards the surface of the bath toform a secondary reaction zone in a section of the space above the baththrough which oxidisable products released from the primary reactionzone flow to reach the waste gas outlet in the vessel and in which theoxidisable products are oxidised and the heat released by such oxidationis transferred into the bath.

It is understood that references herein to "a bath of molten metal"cover a bath containing molten metal and slag as well as a bathcontaining molten metal only.

The present invention is based partly on the realisation that a moltenmetal bath provides a suitable environment, both in terms of temperatureand composition, for converting inorganic solid waste, particularlydusts, into more readily disposable components. The present invention isalso based partly on the realisation that the use of a secondaryreaction zone for oxidising any oxidisable products released from themolten metal bath provides a means of minimising the energy input tomaintain the temperature of the molten metal bath.

It is preferred that the method further comprises injecting a gas intothe bath to cause splashes and/or droplets of molten metal to be ejectedupwardly from the bath into the secondary reaction zone or into asection of the space above the bath which is between the secondaryreaction zone and the waste gas outlet to facilitate efficient heattransfer to the bath and scrubbing of volatilised species and anyparticulate material in the products released from the primary reactionzone and/or produced in the secondary reaction zone.

It can readily be appreciated that the combination of the oxidation ofany oxidisable products in the secondary reaction zone and the scrubbingeffect provided by the splashes and/or droplets of molten metal in thesecondary reaction zone or downstream thereof provides a high level ofassurance against unreacted or partially reacted inorganic solid wasteshort-circuiting treatment altogether and reporting in the exit gasstream from the vessel. This is achieved by providing at least twoseparate reaction zones through which unreacted or partially reactedinorganic waste must pass before exiting the vessel.

It is particularly preferred that the method further comprises injectingcarbonaceous material into the bath to form a carburising zone in whichthe carbon in the carbonaceous material dissolves into the bath and isavailable for reaction with waste in the primary reaction zone.

The term carbonaceous material is herein understood to include: solidcarbonaceous fuels such as coke and coal; liquid fuels such as oil,light fuel oil, diesel oil and heavy fuel oil; and gaseous fuels, suchas natural gas, methane, ethane, propane, butane; or any mixtures of thefuels.

It is preferred that the carbonaceous material be selected from one ormore of the group comprising coal, spent pot linings from aluminiumsmelting furnaces, and sewage sludge. It is particularly preferred thatthe carbonaceous material comprises coal.

In the above described embodiment the heat transferred to the bath fromthe secondary reaction zone contributes to balancing the heat loss fromthe bath as a consequence of endothermic reactions in the carburisingand primary reaction zones.

One particularly preferred embodiment comprises locating the carburisingzone directly below the secondary reaction zone.

It is preferred that the bath comprises at least 10% metal. It isparticularly preferred that the bath comprises at least 70% metal. It ismore particularly preferred that the bath comprises at least 80% metal.

It is preferred that the metal be selected from one or more from thegroup comprising iron, ferroalloys, nickel, tin, chromium, silicon, andcopper, and mixtures thereof. It is particularly preferred that themetal comprises iron.

It is preferred that the gas injected into the bath to cause moltenmetal and slag splashes and/or droplets to be ejected upwardly into thesecondary reaction zone be selected from one or more of an inert gas,recycled process gas, natural gas, CO₂, propane, or butane, or mixturesof the gases. It is particularly preferred that the inert gas benitrogen.

It is preferred that the oxygen-containing gas be selected from thegroup comprising oxygen, air and steam. It is particularly preferredthat the air be preheated. It is more particularly preferred that theair be preheated to temperatures in the range of 900° to 1600° C.

The present invention is described further with reference to theaccompanying FIGURE which is a schematic illustration of a preferredembodiment of a method of treating inorganic solid waste in accordancewith the present invention.

The preferred embodiment of the method of the present invention isdescribed hereinafter in the context of treating dusts although it isunderstood that the present invention is not so restricted and extendsto the treatment of solid inorganic waste generally.

The preferred embodiment of the method is carried out in a vesselgenerally identified by the numeral 3.

The vessel 3 may be of any suitable known design of metallurgical vesselwith refractory lined internal walls and an outer metal shell. In thearrangement shown in the FIGURE the vessel 3 is a generally cylindricalshape disposed horizontally and has bottom tuyeres 5, 7, a slag/metaltap 9, an air injection port 10, and an upper off-gas outlet 11 at oneend of the vessel 3. Typically, the ratio of the length and the diameterof the vessel is 3:1.

The vessel 3 contains a volume of molten metal which comprises at least10% iron and a layer of slag at a temperature of 1400° C. The othermetals in the bath may be selected as required and, by way of example,may comprise one or more of ferroalloys, tin, nickel, silicon andcopper.

The preferred embodiment of the method comprises injecting dustsentrained in a suitable carrier, such as an inert gas, through thebottom tuyeres 5 into the bath to form a primary reaction zone indicatedschematically by the line identified by the numeral 13 which is locatedat the end of the vessel 3 remote from the off-gas outlet 11. The dustsundergo a range of reactions and phase changes in the primary reactionzone 13 depending on the composition of the dusts. Typically, the metaloxides in the dusts are reduced and the metal values report into thebath or in some cases are volatilised. Other components of the dusts maybe broken down or volatilised and released directly into the gas spaceabove the bath.

The method also comprises injecting pre-heated air, typically at atemperature in the range of 900° to 1600° C., or any other suitableoxygen-containing gas through injection port 10 towards the surface ofthe bath adjacent the primary reaction zone 13 to form a secondaryreaction zone indicated schematically by the line identified by thenumeral 17 in the section of the space above the bath that is locatedbetween the section that is directly above the primary reaction zone 13and the off-gas outlet 11.

The method also comprises simultaneously injecting nitrogen or my othersuitable gas through tuyeres 7 into the bath immediately below thesecondary reaction zone 17 to cause eruption of molten metal and slag insplashes and/or droplets from the surface of the bath into the secondaryreaction zone 17. Typically, the nitrogen is injected in an amountgreater than or equal to 0.1 Nm³ min⁻¹ tonne⁻¹ of molten metal in thebath.

In the secondary reaction zone 17 the pre-heated air oxidises anyoxidisable products from the primary reaction zone 13. Furthermore, theheat released by such oxidation is efficiently transferred to thesplashes and/or droplets of molten metal and slag and subsequently intothe bath when the splashes and/or droplets fall downwardly to thesurface of the bath. The splashes and/or droplets also scrub volatilisedspecies and any particulate material from the primary reaction zone 13and/or formed in the secondary reaction zone 17 and transfer thescrubbed values to the bath.

It is preferred that the carbonaceous material be selected from one ormore of the group comprising coal, spent pot linings from aluminiumsmelting furnaces, and sewage sludge. It is particularly preferred thatthe carbonaceous material comprises coal.

It is noted that in effect the splashes and/or droplets of molten metaland slag form a curtain which is an effective and efficient means oftransferring heat to the bath and scrubbing volatilised species andparticulate material from products from the primary reaction zone 13and/or secondary reaction zone 17.

Typically, the temperature in the secondary reaction zone 17 iscontrolled to be at least 200° C. higher than that of the molten metal.Typically, the temperature in the secondary reaction zone 13 variesbetween 1500° C. and 2700° C.

It can be readily appreciated from the foregoing that in the preferredembodiment of the method the secondary reaction zone 17 has threeimportant functions. Specifically, the secondary reaction zone 17:

(a) oxidises any oxidisable products from the primary reaction zone 13;

(b) ensures that the heat released by such oxidation is transferred tothe bath; and

(c) scrubs any volatilised species and any particulate material from theprimary reaction zone 13 and/or formed in the secondary reaction zone17.

The preheated air may be injected into the secondary reaction zone 17 byany suitable means such as top-blowing single or multiple tuyeres orlances with one or more openings.

In many instances, the reduction of metal oxides in the dusts to metalvalues will be a dominant reaction in the primary reaction zone 13. As aconsequence, in such situations, in order to maintain a level of carbonin the bath to reduce efficiently the metal oxides in the dusts, themethod also comprises injecting carbonaceous material such as coal intothe bath through tuyeres 7 to form a carburisation zone indicatedschematically by the line identified by the numeral 15. The volatiles inthe coal are thermally cracked and the carbon dissolves in the iron anddisperses through the bath and in particular into the primary reactionzone 13.

It is noted that the heat transfer to the bath is important sincereduction reactions in the primary reaction zone 13 and thecarburisation zone 15 are essentially endothermic and it is important tobalance the heat loss due to such reactions to maintain the temperatureof the bath at an effective operating level.

It can be readily appreciated from the foregoing that the preferredembodiment of the method of the present invention is an efficient meansby which solid inorganic waste, particularly dusts, can be convertedinto component parts which are non-hazardous and comparativelystraight-forward to recover.

In addition, it can be readily appreciated that the use of two separatereaction zones in the preferred embodiment provides a high level ofassurance against unreacted dusts, which may include hazardouscomponents, short-circuiting treatment altogether.

Many modifications may be made to the preferred embodiment of the methodof the present invention without departing from the spirit and scope ofthe present invention.

In this regard, whilst in the preferred embodiment the inorganic solidsand coal are injected into the bath to form separate, essentiallymacro-sized, reaction and carburisation zones in the bath, it canreadily be appreciated that the present invention is not so limited andthe injection of the constituents into the bath can be controlled toform arrays of separate essentially micro-sized primary reaction andcarburisation zones.

Furthermore, whilst the preferred embodiment includes the location ofthe secondary reaction zone 17 immediately above the carburisation zone15, it can readily be appreciated that the present invention is not solimited and the secondary reaction zone 17 may be located above asection of the bath that is adjacent to the carburisation zone 15.

Furthermore, whilst the preferred embodiment comprises injectingnitrogen or any other suitable gas into the bath to cause eruption ofmolten metal and slag splashes and droplets to form a curtain in thesecondary reaction zone 17, it can readily be appreciated that thepresent invention is not so limited. By way of example, the curtain ofsplashes and droplets of molten metal and slag may be projected into asection of the bath which is between the secondary reaction zone 17 andthe waste gas outlet 11 so that products, gaseous or solid, flowing fromthe secondary reaction zone 17 are required to pass through the curtainbefore reaching the waste gas outlet 11. As a consequence, the curtainenables heat transfer back to the bath and scrubbing of volatilisedspecies and any particulate material flowing from the secondary reactionzone 17.

We claim:
 1. A method of treating inorganic solid waste in a bath ofmolten metal in a vessel which has a bottom, top, space above the bathand a waste gas outlet, the method comprising:(a) injecting said wasteand a carbonaceous material into said bath to form a primary reactionzone in which there are reactions between the waste and the carbonaceousmaterial in the bath; (b) injecting oxygen-containing gas towards thesurface of the bath to form a secondary reaction zone in a section ofthe space above the bath through which oxidizable products released fromthe primary reaction zone flow to reach the waste gas outlet, oxidizingthe oxidizable products with the heat released by such oxidation istransferred into the bath; and (c) injecting a gas into the bath tocause splashes and/or droplets of molten metal to be ejected upwardlyfrom the bath to form a curtain of splashes and/or droplets traversingthe space between the surface of the bath and the top of the vessel, thecurtain extending from the surface of the molten bath to the top of thevessel, the molten metal ejected upwardly into the secondary reactionzone or into a section of the space above the bath which is between thesecondary reaction zone and the waste gas outlet to facilitate efficientheat transfer to the bath and scrubbing of particulate material in theproducts released from the primary reaction zone and/or formed in thesecondary reaction zone.
 2. The method defined in claim 1 furthercomprising injecting solid carbonaceous material into the bath to form acarburising zone in which the carbon in the carbonaceous materialdissolves into the bath, said solid carbonaceous material beingavailable for reaction with waste in the primary reaction zone.
 3. Themethod defined in claim 2, wherein the carbonaceous material is selectedfrom one or more of the group comprising coal, spent pot linings fromaluminium smelting furnaces, and sewage sludge.
 4. The method defined inclaim 3, wherein the carbonaceous material comprises coal.
 5. The methoddefined in claim 2, further comprising locating the carburising zonedirectly below the secondary reaction zone.
 6. The method defined inclaim 1, wherein the bath comprises at least 10% metal.
 7. The methoddefined in claim 6, wherein the bath comprises at least 70% metal. 8.The method defined in claim 7, wherein the bath comprises at least 80%metal.
 9. The method defined in claim 6, wherein the molten metal isselected from the group consisting of nickel, tin, chromium, silicon,copper, and mixtures thereof.
 10. The method defined in claim 1, whereinthe gas injected into the bath to cause molten metal and slag splashesand/or droplets to be ejected upwardly into the secondary reaction zoneis selected from the group consisting of nitrogen, recycled process gas,natural gas, CO₂, propane, or butane, and mixtures thereof.
 11. Themethod defined in claim 1, wherein the oxygen-containing gas is selectedfrom the group comprising oxygen, and air.
 12. The method defined inclaim 11, further comprising the step of preheating theoxygen-containing gas.
 13. The method defined in claim 12, wherein theoxygen-containing gas is preheated to temperatures in the range of 900°to 1600° C.